Benthic Animal Invasion of Land: Ichnological Evidences from the Early Devonian Lianhuashan Formation, South China

Early Devonian was a critical evolutionary period in the process of mid-Paleozoic terrestrialization. The Lower Devonian Lianhuashan Formation of the Liujing section contain abundant and diverse trace fossils, providing key evidences for the benthic animal invasion of land. Here, we document seven ichnogenera from the lower part of the Lianhuashan Formation in the Liujing section, Guangxi, South China. These are Cruziana, Palaeophycus, Planolites, Lockeia, Rusophycus, Skolithos, Treptichnus, illustrating a depauperate CruzianaIchnofacies with five feeding strategies. Sedimentological and ichnological analysis suggests that the depauperate Cruziana Ichnofacies is present in the transition from estuarine to tide-dominated coastal marine settings. Detailed analysis of the morphological and functional characteristics of the trace fossils revealed that Cruziana assemblages demonstrated the efficient feeding strategies and complex behavioral habits of benthic animals (e.g., Arthropods, Polychaetes) in the Early Devonian. The symbiotic relationships of trace fossils reflect the ecological diversification among the shallow-tier crawling, infaunal foraging and suspension feeding. Under the influence of the Caledonian movement, significant changes occurred in the marine-terrestrial settings in South China. The ichnofauna in the Lianhuashan Formation recorded the gradually transition from an extremely estuarine to normal coastal marine settings. This study provides important sedimentological and paleoecological evidence for the adaptive evolution of benthic animals during the process of the mid-Paleozoic terrestrialization.